Legalization of for example weed certainly has not increased the amount of shady dealers.

Why on earth would I go through the effort of finding a dodgy dealer if I can just walk to the nearest regulated clean coffee shop(weed shop) and get some that have been checked, plus I can get my money back if there is a problem.

Same for sex work, the only shady dealings would be people looking for intentionally abusive sex that would touch a shady establishment.